High-strength galvanized steel sheets (hot galvanized iron, HGI), in which a high-strength hot-rolled steel sheet is used as a base steel sheet, have been widely used as structural materials, etc.
As the high-strength hot-rolled steel sheet which is a base steel sheet of the high-strength galvanized steel sheet, steel species typically containing Nb have been used.
The high-strength hot-rolled steel sheet is produced by heating a steel slab typically containing Nb and hot-rolling it in an austenite region of Ar3 or more, followed by coiling.
However, when the steel slab containing Nb is hot-rolled in the austenite region of Ar3 or more as described above, Nb delays recrystallization on hot-rolling, so that a rolling load of the finishing rolling is increased, and accordingly there is a problem that by generating the rolled surface roughness, poor threading performance and surface defects, particularly, defects such as sand type scales, of the steel sheet occur.
As conventional techniques for improving such surface defects, particularly scale defects, methods for improving scale defects by increasing the number of injections of cooling water, decreasing bar thicknesses or strengthening FSB (finishing scale breaker) conditions, when performing descaling in front of rough rolling, and the like have been known.
However, since the conventional techniques cause hot-rolled threading miss rolls and size changes frequently, they cannot be considered as a fundamental solution.
Therefore, there is a demand for a technique capable of providing a hot-rolled steel sheet, particularly a hot-rolled steel sheet for a galvanized steel sheet, having excellent surface characteristics by solving the problem of surface scale defects without operating problems.